I'm not 100% sure, but from what I can see, a thread that is tagged as a question, but has replies will have an "answered" mark, but without the check. If the OP comes to select one or more of the replies as his/her preferred answer, a check mark is added.
Edit: Xenthos has the right of it. Threads in the Q&A section that don't have an "answer" tag are threads not marked as questions when they were made.
Finally, you can move your own threads. After you post them, go to the Common Grounds forum index, and hover your mouse over your thread. The settings gear-sign should pop up. Click that and click edit, then in the edit screen, select the new section you want your thread to be and click save.
When you post a question in the Q&A section, mark it as a "Question" (so the little label on it specifically says "Question").
Whenever anyone replies to that post, it switches to "Answered," but without a checkmark. This lets everyone else know that someone has responded to the query, but the answer has not yet been accepted as fully satisfying the questioner.
The person who made the post can then either ignore the answer, mark it as having answered their question ("best answer"), or mark it as being a waste of time (setting the post's flag back to "Question" again).
I've also seen more than one response get marked as the answer, so it doesn't appear to really be a "best answer" thing, you can select responses that together comprise the best answer.
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